1. Field of the Invention
The invention relates to a digital video effects system for producing moving effects.
More particularly, the invention relates to a digital video effects system comprising a digital video effects apparatus for manipulating an input image to generate an effect and control means for controlling the apparatus to generate an output image sequence comprising a sequence of output images at respective output image timings in which different manipulations are applied for successive images such that the effect changes with time.
2. Description of the Prior Art
The result of applying different manipulations for successive images produces motion within the output image sequence. Examples of such motion could be the translation or the rotation of the input image in the output image sequence. This, however, is only one type of motion which can be associated with an image sequence output by a digital video effects system. Where the input to the digital video effects system is a sequence of input images there can be movement within that input image sequence itself. Typically this will be natural motion captured via a camera as a result of movement of the camera or movement of the subject being captured.
A visual recording of motion comprises a sequence of samples of the original scene. The precise nature of the sampling will depend on the type of camera used. In the case of a traditional film camera the recording is made by alternating between exposing a frame of the film with an open camera aperture and moving the film forward with the camera aperture closed. In such a case, each exposure comprises an integration of the motion over the period during which the aperture is open. The effect of this integration is to give a blurring of the image. The degree of blurring depends on the speed of movement of the object concerned, and the shutter aperture time, which is itself a function of the camera mechanism, the sensitivity of the film recording medium and the required artistic effect. As the aperture time is decreased, so the moving image will become less blurred and the motion portrayal will become more strobe-like.
The situation is different with a video camera, however, as video is based on a scanning technique, normally an interlaced scanning technique. For interlaced scanning, each video frame, which comprises a certain number of scan lines, is comprised of two field scans, in the first of which alternate lines are scanned and in the second of which the remaining lines are scanned. The image integration time depends on the camera technology used. In the case of a tube camera with a photo-voltaic cell and a scanning electron beam which scans over the cell tracing out the scan lines for the first field followed by the second field, the integration time for each field is approximately one frame time with the integration periods for each field being overlapped by half a frame time (i.e. one field time). These differences have an effect which can be perceived when viewing a recorded moving picture. For example, if the wagon wheels on a stage coach appear to be rotating in the opposite direction to the direction of travel or at a different speed to that expected, one can surmise that this picture has been recorded on film because of the stroboscopic nature of that recording technique.
Video cameras based on Charge Coupled Device (CCD) technology allow more flexibility. The integration period for a CCD operating in a simple field mode corresponds to that for a camera tube (i.e. the integration period for each field is one frame) with the periods for alternate fields being overlapped by 50%. However, it is possible to select the integration period by electronically shuttering the CCD. In this way it is possible in some cases to make wagon wheels appear to rotate backwards to emulate a film recording or to generate special effects.
For the type of motion which is produced in a digital video effects system by changing the manipulation applied for successive output images to form an output image sequence, there is, in effect, no integration period. In other words each new image is generated instantaneously. If, therefore, the input image is translated or rotated within the output image this will have the appearance of a rather unnatural, highly temporarily aliased motion. If it is desired to produce natural looking image sequences, this temporal aliasing is often undesirable. This effect can be particularly disturbing where the source image material is a video image manifesting the integration effects described above.